(a) Field of the Invention
The present invention relates to an improvement on the method for dehydration treatment of sludges containing organic matters, such as raw sludge, digested sludge, surplus sludge, aggregated sludges and mixed sludge arising from sewage or industrial waste water treatment facilities.
(b) Description of the Prior Art
Heretofore, the aforementioned various sludges containing organic matters have been subjected to dehydration treatment by means of a centrifugal dehydrator, a vacuum dehydrator or a pressure dehydrator with adding thereto a dehydrating agent such as high molecular weight organic polymers, an iron salt or slaked lime. These methods of dehydration in the prior art, however, are attended with various troubles such that it is difficult to make the water content of the sludges, per se, to be less than 85% by weight, and when an iron salt or slaked lime is employed, the apparent water content of the cakes can be decreased to 85% by weight or less, but on the other hand the amount of the cake increases sharply. Meanwhile, in view of the necessity of preventing environmental pollution, the art of treating water and waste water has advanced year after year, and the rate of recovery of organic matters or suspended substances from water and waste water has been much improved. Consequently, the amount of the resulting sludges, particularly sludges which are hard to dehydrate, has increased year after year, posing a grave social and environmental problem.
Sludges, especially sludges which are hard to dehydrate, generally contain a lot of organic matters, and these organic matters are reported to be composed of high molecular proteins and polysaccharides. With attention being paid to the organic matters contained in these sludges, various attempts have been made to improve the dehydratability of the sludges, for instance, a method comprising lowering the pH value of a sludge slurry to the vicinity of the isoelectric point of the proteins, a method comprising pouring an oxidizer together with air or oxygen continuously into the sludge at a temperature of 80.degree. C., a method comprising subjecting the sludge to heat treatment at a medium high temperature, say, 140.degree. to 180.degree. C., or a method comprising refrigerating the sludges so as to freeze the water in the surroundings as well as the inner part of the sludge and melting them thereafter, thereby performing dehydration.